Ultrasonic gas flow meters measure the gas flow by means of ultrasonic signals. For this purpose an ultrasonic gas flow meter has one or more acoustic paths, each path being defined between a pair of ultrasonic transducers. Each transducer of such a pair is capable of transmitting signals to, and receiving signals from the other transducer. When operating, both transducers in turn transmit and receive, resulting in an acoustic signal that travels along the acoustic path—either parallel or at an angle—inside the meter body with the flow direction (downstream) and against the flow direction (upstream). The difference in travel time for both transmission directions is proportional to the gas velocity. In order to measure even low gas velocities accurately, the travel times itself or the difference in travel time need to be measured with very high accuracy and resolution in time.
Typically, ultrasonic gas flow meters operate using a number of short bursts of high frequency signals (for example 5 cycles long). Typically, the operating frequency of ultrasonic transducers is in the range between 50 kHz and 500 kHz. As in ultrasonic gas flow meters differences in travel times need to be measured extremely accurately, a very high definition or resolution in the travel time measurement is a must. This cannot be achieved by using the envelope of the received burst signal, even if this is a short one. In order to obtain sufficient resolution, the exact time of the zero crossings in the burst signal is required. As typically multiple zero crossings are present in the burst signal, this creates the problem of identifying and locating one or more specific zero crossings, to be used as reference point(s) for the travel time measurement. This may be resolved with the aid of the shape of the envelope of the burst signal.
In a gas pipeline, ultrasonic noise may be emitted from various other sources. The frequency spectrum of such noise may extend into the frequency range where the ultrasonic transducers operate. Such noise may be flow induced noise, or equipment induced noise, such as noise emitted by pressure reducing valves. The latter type of interfering noise is more severe, especially when such pressure reducing valves operate at high pressure differentials, for example above 10 or 20 bar. Under these circumstances the amount of dissipated energy can be quite substantial. Even if only a very small fraction thereof is converted into acoustic energy, still a serious amount of noise may be emitted.
The noise from such valves may be characterised by having some dominant peaks at discrete (low) frequencies, but more appropriately it is described as broadband noise. The envelope of the frequency spectrum typically has a maximum in the range between 30 kHz and 80 kHz. This indicates that even outside this frequency range substantial amounts of ultrasonic energy may be present.
The same holds for valves that have been designed to be so-called “silent valves”. The word “silent” typically applies to the noise in the audible range. Sometimes reduction of noise in the audible range is achieved by design modifications that shift the emission of noise to higher frequencies. As this noise is broadband by nature, it easily extends into the frequency range where ultrasonic transducers for ultrasonic gas flow meters may operate.
Due to practical limitations the power of the signals used in an ultrasonic flow meter is restricted to certain levels. The problem arises that either the noise interferes with the ultrasonic signals of the flow meter, or the ultrasonic signals of the flow meter may even become completely buried or masked by the noise. As a result of this the ultrasonic signals used by the flow meter may become undetectable and therefore the meter may become un-operational.
Until now in various ways, attempts have been made to resolve the problem of acoustic noise interfering with the signals of an ultrasonic flow meter.
Attempts have been made to resolve the problem of acoustic noise interfering with the signals of an ultrasonic flow meter by separating the ultrasonic gas flow meter and the noise source spatially. This means separating the two by applying a long length of pipe, preferably also including elbows and T-bends, between the meter and the noise source. As available space does not always allow for such an approach, this method may not only be costly but often is impractical as well.
Moving the operating frequency of the flow meter away from the frequency range of the ultrasonic noise has also been attempted. Due to the increase of attenuation of the ultrasonic signal in the fluid with increasing frequencies, for practical application the usable frequency range is limited to a maximum of about 300 to 500 kHz. This puts a limit on the gain that can be achieved in this way.
Special silencing devices have been designed to separate the noise source and the ultrasonic flow meter respective to each other. The effectiveness of such devices is limited as these devices themselves may generate noise (flow induced noise) at higher gas velocities, and again considerable costs may be involved.
Also attempts have been made to apply a variety of signal processing techniques in order to improve the signal-to-noise ratio, described as filtering, averaging, stacking and correlation.
Applying filtering techniques in the frequency domain does not provide much improvement as the frequency spectrum of the noise often overlaps with the operating frequency of the ultrasonic flow meter. Typically also the ultrasonic transducers are operated in a resonating mode in order to achieve the highest efficiency, consequently the transducers themselves act as frequency selective devices. Therefore, using additional electronic circuitry or software for filtering the signal in the frequency domain will not be very effective.
Improvement of the signal-to-noise ratio may be achieved by increasing the amount of ultrasonic energy by extending the duration of the burst signal. This suffers from the problem that it becomes more difficult to uniquely identify and locate a specific zero crossing with the aid of the shape of the envelope, or suffers from ambiguity when phase detection techniques are used, since the travel time to be measured represents multiple periods of the signal and the phase shift is multiple times 2π.
Another known method for improving the signal-to-noise ratio is referenced as stacking. This means averaging the signal that is obtained by repeatedly sending and receiving an ultrasonic signal. The assumption is that the received burst signals are correlated, such that addition of multiple received burst signals results in an increased signal, while the noise is uncorrelated and therefore will be reduced when multiple received signals are added up. The received signals are added taking the time of the emission as a reference point in order to synchronise the received burst signals while these are being added.
This method as well has limitations because of the natural turbulence in gas flow. The turbulence results in a variability of travel time (also known as “jitter”) as a result of which the received burst signals become uncorrelated. This means that using the time of emission as a reference point, the received signals do not coincide exactly when being added. The averaging or stacking technique is therefore only effective on a short time scale: a time scale short enough, in comparison to the periodicity of the turbulence phenomena, to limit the variances in travel time to values substantially smaller than one period of the frequency in the burst signal. This limits the number of the received burst signals that can effectively be added up in order to increase the signal-to-noise ratio.
In the description of the above method the assumption has been made that each cycle of emitting an ultrasonic burst signal and receiving the same signal is completed before the next one is initiated. A new cycle therefore can only start after the acoustic signal has travelled across the fluid along its acoustic path, which may take up to several milliseconds, especially for a large size meter having long acoustic paths. This limits the number of pulses to the number that can be sent within a specific time interval, i.e. the time that the acoustic signals as received can be considered to be correlated.
As an alternative to this method, the process of sending and receiving of acoustic pulses could be interleaved or overlapping, meaning that acoustic pulses (burst signals) are being fired while previous acoustic pulses are still travelling trough the gas. As a result of this, a sequence of acoustic pulses will arrive at the receiver. The problem with this alternative method, however, is to identify each of the pulses from a sequence when they arrive, as they may be heavily distorted by the acoustic noise, or even buried or masked by the acoustic noise. Averaging the individual burst signals within a sequence using the known repetition rate of the firing of the pulses, will help to recover the pulse from the noise and make the pulses detectable, but results in ambiguity. This means that the burst signal may be retrieved at various points in time, while not being able to detect which one actually represents the travel time.
Correlation techniques are general-purpose signal processing tools that can be applied to a variety of electronic measurements. Major uses are the detection of the presence and the location of signals buried in noise. The waveform of the signal is generally known, for example the signal transmitted by an ultrasonic transducer. The correlation process incrementally slides the reference waveform over the signal being processed, looking for a matching signal. Signals that are not related to the reference waveform result in a correlation value of 0. If a matching signal is found, the correlation value increases to a maximum value of 1 for a perfect match and −1 for a matching but inverted waveform. The maximum value of the correlation function serves both purposes. If the maximum value is close to 1 the desired signal is present, and the location of the maximum is a measure of the propagation delay (or travel time).
The application of the correlation function to the travel time measurement in an ultrasonic meter suffers from several drawbacks. It is computationally intensive (i.e. it requires a great deal of processing power), and considering the very high time resolution that is required for the travel time measurement, it requires very high sampling rates.